summaryrefslogtreecommitdiff
path: root/theories/Wellfounded/Union.v
blob: fbb3d9e3c9cb3ee81199f337b2923c47ece9d35b (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
(************************************************************************)
(*  v      *   The Coq Proof Assistant  /  The Coq Development Team     *)
(* <O___,, * CNRS-Ecole Polytechnique-INRIA Futurs-Universite Paris Sud *)
(*   \VV/  **************************************************************)
(*    //   *      This file is distributed under the terms of the       *)
(*         *       GNU Lesser General Public License Version 2.1        *)
(************************************************************************)

(*i $Id$ i*)

(** Author: Bruno Barras *)

Require Import Relation_Operators.
Require Import Relation_Definitions.
Require Import Transitive_Closure.

Section WfUnion.
  Variable A : Type.
  Variables R1 R2 : relation A.

  Notation Union := (union A R1 R2).

  Remark strip_commut :
    commut A R1 R2 ->
    forall x y:A,
      clos_trans A R1 y x ->
      forall z:A, R2 z y ->  exists2 y' : A, R2 y' x & clos_trans A R1 z y'.
  Proof.
    induction 2 as [x y| x y z H0 IH1 H1 IH2]; intros.
    elim H with y x z; auto with sets; intros x0 H2 H3.
    exists x0; auto with sets.

    elim IH1 with z0; auto with sets; intros.
    elim IH2 with x0; auto with sets; intros.
    exists x1; auto with sets.
    apply t_trans with x0; auto with sets.
  Qed.


  Lemma Acc_union :
    commut A R1 R2 ->
    (forall x:A, Acc R2 x -> Acc R1 x) -> forall a:A, Acc R2 a -> Acc Union a.
  Proof.
    induction 3 as [x H1 H2].
    apply Acc_intro; intros.
    elim H3; intros; auto with sets.
    cut (clos_trans A R1 y x); auto with sets.
    elimtype (Acc (clos_trans A R1) y); intros.
    apply Acc_intro; intros.
    elim H8; intros.
    apply H6; auto with sets.
    apply t_trans with x0; auto with sets.

    elim strip_commut with x x0 y0; auto with sets; intros.
    apply Acc_inv_trans with x1; auto with sets.
    unfold union in |- *.
    elim H11; auto with sets; intros.
    apply t_trans with y1; auto with sets.

    apply (Acc_clos_trans A).
    apply Acc_inv with x; auto with sets.
    apply H0.
    apply Acc_intro; auto with sets.
  Qed.


  Theorem wf_union :
    commut A R1 R2 -> well_founded R1 -> well_founded R2 -> well_founded Union.
  Proof.
    unfold well_founded in |- *.
    intros.
    apply Acc_union; auto with sets.
  Qed.

End WfUnion.